Flechtner F.M., Gruber Th., G?ntner A., Mandea M., Rothacher M., Sch?ne T., Wickert J. (Eds.) System Earth via Geodetic-Geophysical Space Techniques

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514 M. Mandea et al.

Fig. 21 Power spectra of for the GRIMM, xCHAOS and MF5 models at the Earth’s reference

radius for SH degrees 16–60

Figure 22 present the vertical down component of the GRIMM lithosphere ﬁeld

modeled at the Earth’s surface for SH degree 16–45 and its differences relative to

MF5. The shapes of these spurious anomalies are typical of those introduced by

along track ﬁltering.

4.2 Lithospheric Field and Its Small Scale

4.2.1 World Digital Magnetic Anomaly Map

A possibility to recover the lithospheric ﬁeld with a shorter wavelength content

(degrees/orders higher than 130) is to merge satellite data, aeromagnetic and marine

survey data, and ground-based data (observatory and repeat stations). They have

also to be processed together. A huge international effort has been done to pro-

duce such a description of the lithospheric ﬁeld, in the frame of the World Digital

Magnetic Anomaly Map (WDMAM).

The GFZ contribution to the WDMAM

project (Hamoudi et al., 2007) is known as the GeoForschungsZentrum Anomaly

Magnetic MAp (GAMMA).

The distribution of available magnetic datasets used to build GAMMA is shown

in Fig. 23. These datasets consist in more than 50 years of aero-magnetic surveys,

research vessel magnetometers crossing the s eas, supplemented by anomaly values

obtained from oceanic crustal ages and observations from satellites. The coverage

is uneven between the continental and ocean areas, as well as between Northern and

Southern hemispheres.

http://projects.gtk.ﬁ/WDMAM/

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Fig. 22 Map of the vertical down component of the GRIMM lithospheric ﬁeld for SH degrees

16–45 at the Earth’s reference radius (top). Map of the vertical down component of the differ-

ences between the GRIMM and MF5 models for SH degrees 16–45, at the Earth’s reference radius

(bottom)

The available magnetic compilations are i n various formats and projections.

Before applying any ﬁltering or merging procedures, all grids have to be trans-

formed from their local reference system to a common representation on the global

reference ellipsoid, WGS84. Marine data have been reprocessed to produce grids at

mean sea level, when possible.

The continental-scale aeromagnetic compilations have been produced after

stitching together different regional anomaly grids. These regional surveys were

ﬂown at various epochs and hence use various core ﬁeld models to reduce the

observed data. The patch-worked grids prone to errors, mismatch in anomaly shapes

and anomalous intensity values clearly noticeable across overlapping regions, intro-

duce ﬁctitious anomalies. The lack of absolute reference makes it difﬁcult to

recover the long wavelength information. Despite their decimation on a regular

516 M. Mandea et al.

Fig. 23 Data distribution: from aeromagnetic and marine surveys (light grey), satellite observa-

tions (dark grey) and oceanic crustal ages (white)

grid, the ﬁnal resolution may not be uniform and only a spectral analysis such as

wavelet techniques could help in precisely locating the regions of varying reso-

lution. Furthermore, not only the core ﬁeld models used to reduce these regional

surveys are mostly unknown, but also each individual compilation panel was often

reduced using a local polynomial. This lack of information prevents us from fur-

ther correcting the grids for a more homogeneous core ﬁeld model. According to

the WDMAM recommendations, the grids are resampled to 0.05

◦

(≈ 3



or 10 km

wavelength at the equator).

As an example of the difﬁculties that occurred during the merging process-

ing, detailed anomaly features are compared over overlapping regions between two

adjacent grids. The example shown in Fig. 24 concerns the European (EUR) and

Eurasian (ERS) grids. The EUR grid shows anomalies that are smoother than the

neighboring ERS grid. Two reasons may be invoked. Firstly, the EUR grid is upward

continued to 3 km altitude, while the ERS grid is produced by combining many dis-

parate aeromagnetic surveys of smaller dimension without upward continuing the

ﬁnal grid. Secondly, a spectral analysis carried out on the EUR grid showed sudden

power decay for wavelengths lower than 30 km and it seems that EUR grid was

ﬁltered in order to produce a homogeneous resolution map. The anomaly features

across the boundary of EUR and ERS grids show partial disagreement of anoma-

lies. For instance, the anomaly feature across the regions marked by a circle in

Fig. 24 appears to continue across the grid edges but the strength of the anomaly

over the EUR grid appears weaker. On t he three indicated proﬁles (P1, P2 and P3)

the strength of the total ﬁeld intensities across the two grids have been investigated.

All proﬁle sections show intensity variations of the ERS grid that are consistent with

The Earth’s Magnetic Field 517

Fig. 24 Map of the two aeromagnetic compilations, European (EUR) and Eurasian (ERS) grids.

The detailed anomaly features between the two grids show differences in their shape, size and

strength. On the three indicated proﬁles (P1, P2 and P3) the strength of the total ﬁeld intensities

across the two grids have been investigated

anomaly patterns of the EUR grid. However, within the overlapping region total ﬁeld

intensities disagree in strength by 10 – 200 nT. At the grid boundary these intensi-

ties vary by 100 – 150 nT, highlighting once again the differences in the anomaly

strength across the two grids. Such differences are also observed between some

other grid boundaries (not shown here). To ﬁnd the sources causing differences in

the shape, size and strength of anomalies between various grids, as demonstrated

for European and Eurasian grids, a detailed procedure was followed on to process

the individual grids.

Once all surveys have been merged together, two different techniques are envis-

aged for ﬁltering and smoothing the ﬁnal grid. A ﬁrst option consists in ﬁltering the

grid using Fourier transforms. This procedure is relatively fast but the main draw-

back is that it is a non-potential method, and the coefﬁcients could not be used for

predicting the three components of the magnetic ﬁeld. Another method is based on

the spherical harmonics analysis. The grid is interpolated on the Gauss-Legendre

knots and the parameters up to spherical harmonic degree 500 are obtained by least-

squares (maximum r esolution of 80 km). Above degree 500, the coefﬁcients are

difﬁcult to be obtained as the signal/noise ratio is very low. This is also due to the

data gaps and heterogeneous resolution at the global scale.

518 M. Mandea et al.

A few other international teams have done the same work for processing mag-

netic data, producing candidates for the WDMAM. The proposed grids have been

compared and merged, and the best aspect of each has been preserved for the ﬁnal

compilation. The resulting map was published by CCGM.

The WDMAM gives the essence of the magnetic anomalies at 5 km altitude. Its

release is still too novel to illustrate its scientiﬁc outcomes. In Fig. 25, we may

however recognize the remarkable magnetic features, such as Chicxulub crater,

Bangui anomaly, Thromsberg anomaly, Richat structure, Atlantic ridge, Bay of

Biscay, Sumatra-Java arc or Paris Basin, features discussed in details by Mandea and

Thébault (2007). The WDMAM has already became an important tool for evaluating

competing hypotheses and reconstructions of continental and oceanic assemblies of

most regions of the world, and understanding their geologic evolution at various

spatial scales.

Of course this ﬁrst edition of the WDMAM is subject of improvements. One

direction is to take i nto account data which were not available e.g. of the national or

particular permission reasons, during t he ﬁrst compilation processing. One example

is given by the large efforts to access to the Northern African surveys (Morocco

and Algeria). These data are partly available, and the ﬁrst map of this region is

shown in Fig. 26, which is an expression of the complexity of the Maghrebides

geology.

Fig. 25 Map of the GAMMA model, candidate to the ﬁrst edition of the World Digital Magnetic

Anomaly Map (WDMAM), giving the essence of the worldwide magnetic anomalies at 5 km

altitude. The most important magnetic features are indicated: (1) Chicxulub crater; (2) Bangui

anomaly; (3) Thromsberg anomaly; (4) Richat structure; (5) Atlantic ridge; (6) Bay of Biscay; (7)

Sumatra-Java arc; (8) Paris Basin

The Earth’s Magnetic Field 519

Fig. 26 The magnetic anomaly map over a part of the Northern Africa, obtained from the available

Morocco data (data by courtesy of Ministry of Energy and Mine, Morocco)

Another improvement for the second edition of the WDMAM concerns the

improvement of the magnetic ocean surveys. The magnetic measurements acquired

over oceans and seas since more than half a century are stored by the GEOphysical

DAta System (GEODAS).

Generally, both the original magnetic ﬁeld intensity and

magnetic anomaly values are stored. The magnetic anomaly values are estimated

from the measured intensity data, by subtraction of the core and external magnetic

ﬁelds. Recently, Quesnel et al. (2009a) used the Comprehensive Models (Sabaka

et al., 2004) to properly remove the core and external magnetic ﬁelds from all origi-

nal measurements. Besides, a careful analysis of each data (track by track) has been

needed to correct or remove many shifted values, as well as to reduce the noise

in some tracklines. Comparisons of magnetic anomaly maps before and after the

indicated corrections highlight the improvement in the quality and coherence of the

data. In Fig. 27 a comparison between pre-corrected and corrected data is shown.

http://www.ngdc.noaa.gov/mgg/geodas/trackline.html

520 M. Mandea et al.

Fig. 27 Maps of the magnetic anomalies over the North Atlantic from the original anomaly values

given by the GEODAS data set (top) and from the CM4-corrected and cleaned magnetic values

(bottom)

4.2.2 Some Qualitative Geology

A global magnetic anomaly map is a tool for geologists to identify large-scale

tectonics, especially in places where no regional geology and tectonic maps are

available. A common method of interpretation consists of comparing the anomaly

ﬁeld with geological and tectonic maps. An exhaustive review of magnetic map-

ping applications is out of scope of this chapter, so only a few examples are given,

which have been selected to give a ﬂavor of magnetic anomaly applications and

interpretations at various spatial scales.

The seeﬂoor spreading. The magnetic mapping of ocean ﬂoor is an essential

tool for reconstructing the history of the Earth. Oceanic linear magnetic anoma-

lies are composed of linear segments of polarity separated by a steep gradient that

were recognized since the 1950s, but were lacking a satisfactory explanation. Vine

and Matthews (1963) noticed the pattern similarity between continental vertical

The Earth’s Magnetic Field 521

magnetostratigraphy and horizontal oceanic lineaments. They proposed that seaﬂoor

linear anomalies result from a combination of reversals in the Earth’s magnetic

ﬁeld and seaﬂoor spreading is symmetrical and parallel to ocean ridges crests. As

magma ascends along the ridges it cools and solidiﬁes. The rocks formed acquire

a thermo-remanent magnetization aligned along the ambient magnetic ﬁeld. As sea

ﬂoor spreading continues, the magnetic ﬁeld is recorded in the rocks symmetri-

cally to the mid-ocean ridge, creating these remarkably continuous lines that are

occasionally displaced by perpendicular fracture systems that reveal variations in

the seaﬂoor spreading and tectonic movements (see Fig. 25 and the North Atlantic

features in Fig. 28).

The Beattie Magnetic Anomaly. The origin of the 1,000 km – long Beattie

Magnetic Anomaly (Fig. 29) in South Africa remains unclear and contentious. Key

issues include the width, depth and magnetization of its source. Recently, Quesnel

et al. (2009b) use uniformly magnetized spheres, prisms and cylinders to provide

the simplest possible models which predict the 1 km – altitude aeromagnetic mea-

surements along a proﬁle across this anomaly. The obtained results suggest that two

similarly magnetized and adjacent sources, with a vertical offset, are more likely to

explain the observed magnetic anomaly than a single source. The best model cor-

responds to two highly-magnetized (> 5 Am

–1

) sheet-like prisms, extending from 9

to 12 km depth, and from 13 to 18 km depth, respectively, and with a total width

reaching 80 km. Associated magnetizations seem to be mostly induced, although

a weak remanent component is required to improve the ﬁt. The aeromagnetic data

and their modeling suggest that the geological sources for the Beattie Magnetic

Anomaly are mostly located in the middle crust and may be displaced by a shear

zone or a fault. Contrary to previous models suggesting serpentinized relics of an

Fig. 28 Magnetic mapping of the North Atlantic ocean’s ﬂoor as seen by the WDMAM

522 M. Mandea et al.

Fig. 29 Magnetic anomaly map at 5 km altitude over South Africa. The Beattie Magnetic

Anomaly (BMA), the Southern Cape Conductive Belt (SCCB), Namaqua-Natal Mobile Belt

(NNMB) and the Cape Fold Belt (CFB) are indicated

inferred suture zone of the Natal-Namaqua Mobile Belt, Quesnel et al. (2009b)

propose that granulite-faces mid-crustal rocks within this belt may cause the Beattie

Magnetic Anomaly.

The Java trench. Magnetic anomaly maps can give information about geodynam-

ics at geological time scales. It is thus used to drive the understanding of geologic

hazard but cannot be employed for hazards prediction. The aeromagnetic features

of the region surrounding the Great Sumatra earthquake and tsunami of December

2004 could provide us with a current and historical view of the active subduc-

tion in the region. The subduction can be seen on the gravity anomaly map by an

increase of the gravity ﬁeld along the subduction zone. A lithospheric magnetic ﬁeld

satellite image conﬁrms the compression boundary between the Eurasian and Indo-

Australian plates. Even if this conception is oversimpliﬁed, a subduction zone near

the magnetic equator, like the Sumatra trench, has a typical positive magnetic sig-

nature above the location where the “cold” magnetic ocean crust penetrates into the

relatively non-magnetic mantle. To the contrary, the magnetic anomaly map realized

at sea-level appears noisy and the large scales structures are hidden or distorted (see

again Figs. 25 and 30). One reason is the data distribution which is not homoge-

neous and furthermore no data are available inland. The more geophysical reason is

that a magnetic map made using shipborne and airborne data is mostly sensitive to

shallow and small dimension magnetic materials. In contrast, the far-ﬁeld imaging

using satellite data has a better sensitivity to the deep and large magnetic anomalies.

Both views are complementary and the use of each data depends on the spatial scale

at which the study is realized.

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Fig. 30 Magnetic anomaly map at the Java Trench

5 Conclusions and Outlook

The different topics addressed in this chapter have in common the wish to better

understand the geomagnetic ﬁeld. In spite of its remarkable persistence, the Earth’s

magnetic ﬁeld is continually changing, covering time-scales from seconds to mil-

lion years, when its polarity can reverse. Considerable success has been achieved

in describing the geomagnetic ﬁeld, with its core, lithospheric, ionospheric and

magnetospheric contributions when recent, Ørsted, CHAMP, SAC-C satellite mea-

surements have been used together with ground-based data. For the ﬁrst time, short

time-scales of secular variation have been detected in satellite data. Some other

interesting signatures have been found in CHAMP magnetic data, such as the SGR

1806-20 magnetar.

Although new and sometimes surprising results have been found when magnetic

satellite data have been investigated, the particular combination of the long-term

steadiness and short-term ﬂuctuations of the magnetic ﬁeld, still rises fundamen-

tal questions about the geodynamo, core-mantle boundary, mantle properties, and

geospace environment.

Answering such questions would have both practical and scientiﬁc beneﬁts. A

full understanding of the magnetic ﬁeld will emerge from improvements in mea-

suring the ﬁeld, separating its different contributions with a better spatial and

temporal resolution, developing new mathematical tools to analyze available data.

The upcoming ESA Swarm

mission will provide the best-ever survey of the geo-

magnetic ﬁeld and its temporal evolution. This constellation will also beneﬁt from a

http://www.esa.int/esaLP/LPswarm.html